Composition for descaling ferrous metal



Patented Oct. 5, 1948 UNITED STATES Examine 7 PATENT OFFICE Harold A.Robinson, Midland, Mich., assignor to The Dow Chemical Company, Midland,Mich., a corporation of Delaware No Drawing. Application April 16, 1945,Serial No. 588,711

3 Claims. 1

The invention relates to a method of cleaning the surface of ferrousmetal articles with aqueous acid solutions. It more particularlyconcerns an improved composition of aqueous acid solution possessingespecial characteristics adapting it for the purpose in hand.

Among the various uses of aqueous acid solutions is that of descalingferrous metals. As is well known, such metals normally carry a coatingof scale of one or more of the iron oxides formed during the manufactureor use. In some uses, other substances may be deposited upon the surfaceforming an adherent coating or scale either alone or in admixture withthe aforesaid oxides. In the operation of steam boilers, for example, avariety of scales are deposited on the metal from the water dependingupon its nature and other factors. All such deposits generally areundesirable and may seriously interfere with efiicient use of theferrous metal as a heat transfer medium. In the case of ferrous metalheat-exchange surfaces, the presence of oxidic coatings formed asoxidation products of the ferrous metal are a cause of more or lessinefliclency of heat transfer. There are many other uses of ferrousmetals, the efficiency of which is adversely affected by oxidic scaledeposits.

Many of these scale deposits are soluble in aqueous hydrochloric acidsolutions and usually can be readily removed merely by maintaining for along enough time a sufficient quantity of the acid solution in contactwith the deposit at an appropriate temperature. However, the acidsolution also attacks the underlying metal as it becomes exposed to theacid solution during the removal of the scale. Because of this, aninhibitor of the corrosive action of the acid is usually added to thesolution, thereby reducing the severity of this attack. I have foundthat in spite of having added a conventional corrosion inhibitor, theacid solution may and often does persist in severely corroding theunderlying metal. In investigating the causes of this effect I havefound that there are two more or less independent corrosive factorsusually involved, one of these is the corrosive action of the aciditself, the other the corrosion due to reduction by the ferrous metal ofthe ferric ion of the compounds formed as the acid dissolves ferricconstituents of the scale.. Such a reduction is expressed quantitativelyby the following equation:

Fe 2Fe+++ 3Fe++ (solid metal) dissolved (disso' lved erric iron) ferrousiron) This equation shows that each 2x55.84 pounds of ferric ironconstituent dissolved from the scale is a solvent for 55.84 pounds ofsolid metal underlying the scale. While the conventional inhibitorsexert a retarding effect on the rate of corrosion of the solid ferrousmetal by the acid of the descaling solution, they have little or noeffect in retarding the corrosive action of ferric compounds dissolvedby the acid solution as it attacks the scale. Attempts to descaleferrous metal surfaces having ferric compounds in the scale usingconventionally inhibited hydrochloric acid generally result inundesirable corrosion. In addition, the corrosive effect of thedissolved ferric compounds in the descaling acid solution is acceleratedby the heating usually necessary to facilitate the removal of the ironoxide-containing scale deposits. This heating has the furtherdisadvantage of decreasing the effectiveness of the conventionalinhibitors.

One of the objects of the invention is to provide a h droc ic i solutionsuitable for dissolving from ferrous metal surfaces scale containing aferric ion-producing material.

Another object of the invention is to provide a method of removingferric oxide containing scale deposits from a ferrous metal surface bytreatment with an aqueous solution without significant attack upon theunderlying surface.

Other objects and advantages will appear as the description of theinvention proceeds.

My invention is predicated upon the discovery that by including anoxidizable soluble salt 01 a metal selected fromtlieffciufibf'thdfibh-blating metals consisting of chromium and titaniumin the aqueous hydrochloric acid, together with an acid corrosioninhibitor scale containing ferric ion (Fe+++) producing material, may beremoved from a ferrous metal surface with greatly reduced attackcompared to that obtained when the acid solution and acid corrosioninhibitor are used without the addition of the aforesaid soluble salt ofa non-plating metal. I

The invention, then, consists in the scale removing composition andmethod of removal of scale therewith hereinafter more fully describedand particularly pointed out in the claims.

In carrying out the invention, aqueous hydrochloric acid is used in aconcentration suitable for decomposing, dissolving. or disintegratingthe scale deposit to be removed from the ferrous metal surface, such asthose of iron and steel. Complete solution of the scale is not alwaysnecessary. Some scale deposits contain both acid-soluble and -insolubleconstituents and usually slough off when attacked by the acid sulphataacetate, etc.

without completely dissolving. A concentration between about and percent is usually suitable, although other concentrations can be used. Apreferred concentration is about 15 per cent of hydrochloric acid byweight. Some of the higher concentrations are more diflicult to inhibitagainst acid corrosion; hence, the lowest concentration that willeffectively remove the scale is preferably used.

To minimize the attack of the acid ion (H+ ion) on the ferrous metalunderlying the scale a suitable inhibitor of this action is added to theacid. A very large number of substances and products having the propertyof inhibiting the corrosive action of acid on iron or steel are known,examples of which are found among the organic nitrogen bases, such aspyridine, quin oline, orthotoluidine. In addition, many organic sulphurcompounds possess this property, e. g. the mercaptans, thiourea, andthiocyanates. Some aldehydes, likewise, may be used. Soluble arseniccompounds are also effective. The concentration of the acid corrosioninhibitor to employ depends upon its effectiveness and the extent towhich it is desired to suppress the corrosion due to the acid and thetemperature at which the acid solution is to be used as is wellunderstood in the art. Generally, the amount used is in the range of 0.1to 1 per cent of the weight of the acid solution.

In order to suppress the added corrosiveness acquired by the acidsolution as it dissolves ferric iron from the scale to form ferric ionsin the acid solution I add to it, in addition to the H+ ion corrosionretardant or acid inhibitor, an inhibitor of ferric ion corrosionconsisting of an oxidizable salt of a metal selected from the groupconsisting of the non-plating metals, chromium and titanium. Varioussoluble chromium salts [maybe used, such as the chloride, iodide,acetate sulphate, etc. Similarly, various soluble titanous salts may beused, such as the chloride,

Ih' amount used is preferably that calculated to be in excess of thatapproximately stoichiometrically equivalent to the amount of ferric ironto be dissolved by the acid in descaling the ferrous metal surface.

The stoichiometric proportions for the oxidizable divalent chromium ionand the oxldizable trivalent titanium ion are given, respectively, inthe following equations:

An equal amount or as much as 10 to 50 per cent in excess of theseamounts may be used.

' To determine the amount of ferric ion which will be produced in agiven scale removal operation one may resort to a conventional analysisof the scale giving the percentage of ferric iron therein. From such ananalysis and a knowledge of the area of the ferrous metal surfacecovered by the scale to be removed, an estimate of the total weight offerric ion-forming material that can be dissolved by the acid solutionis computed on the assumption that during the acid treatment of thescale all the ferric ion-forming constituents therein will be dissolvedby the acid solution. The stoichiometrically equivalent weight, orpreferably 10 to 50 per cent in excess thereof, of oxidizable'chromiumor titanium compound is then ascertained from the weight of ferricionforming material in accordance with the weight ratios of the aboveequations.

Similar data as to the amount of acid-soluble material in the scale maybe used to ascertain the quantity of acid solution needed, the amountused being preferably substantially in excess of actual needs. In someinstances in order to reach all the scale, a larger volume may be neededas is usually the case in treating steam boilers. In such cases,suflicient solution is usually used to fill the vessel even though theamount of acid is more than enough to attack the scale.

The following example is illustrative of a mode of carrying out theinvention:

A Water tube steam boiler having a volume of 5000 gallons is to bedescaled. Examination shows the heat transfer surfaces are coated withscale containing ferric ion-forming constituents and other acid-solublematter, the total amount of ferric iron in the scale over the area to bedescaled being about 200 pounds. For treating such a vessel, thesimplest procedure is to prepare enough treating acid to fill theboiler, i. e.- 5000 gallons. About 840 pounds of titanium trichloride(i. e. 50 per cent more than the stoichiometrical equivalent of the 200pounds of ferric iron 0f the ferric iron content of the scale to beremoved) is the amount used in the treating solution. For the acidcorrosion inhibitor about 225 pounds of sodiumarsenite may be used,thereby giving a concentration of about 0.5 per cent in the acidsolution. These two inhibitors are preferably added to the treating acidby first dissolving them in a small volume of 15 per cent hydrochloricacid so as to make concentrated solutions of these inhibitors and thendiluting these strong solutions with enough 15 per cent hydrochloricacid to make 5000 gallons of treating acid solution. After theseadditions have been added to and uniformly mixed with 15 per centhydrochloric acid to make 5000 gallons of mixed solution, the treatingsolution thereby obtained is ready for introduction into the boiler toremove the scale. The boiler is filled with the descaling solution soprepared and it is allowed to remain in contact with the scale thereinfor several hours or until the scale dissolving action ceases. This canbe determined by making periodic titrations for acidity of the solutionwhich declines as the acid decomposes the scale. When titration showsthat the decline of the acid concentration has ceased, the scaleremoving action usually has ceased also. It is advantageous to heat theacid solution during its scale removal action as this accelerates therate of attack, temperatures up to about 175 F. usually may be usedwithout serious attack on the underlying metal.

The boiler is then drained and rinsed with water. If desired to insureremoval of all traces of acid from the boiler, it may be filled with adilute (e. g. 1 per cent) solution of sodium carbonate in water andboiled for a few minutes.

Further illustrations of suitable descaling acid solutions are: (1)those containing about 5-25 per cent of hydrochloric acid, 1.65 to 2.5of titanium trichloride, and 0. o 1. per cent of tially water.

Descaling solutions containing divalent chmmium salts should not beunduly exposed to the? air as they are subject to atmospheric oxidation;

Examiner The descaling solutions containing soluble trivalent titaniumsalts are not so easily oxidized.

As illustrative of the effect on corrosion of employing a non-platingmetal salt in the inhibited acid solution. the following series of testsconducted in a 15 per cent solution of hydrochloric acid are cited. Inthese tests pieces of mild steel 1" x 2" x 54;" were submerged in 300grams of the acid solution for 16 hours while the solution wascontinuously agitated and maintained at 150 F. and the loss in weightdue to attack on the pieces was noted. The weight loss data wasconverted into pounds of metal dissolved per square foot per day givingthe corrosion rates reported in the table below. To ascertain the effectof ferric ion-forming material being present in the acid, 1.33 per centof F8304 by weight was dissolved in the acid solution giving aconcentration of ferric iron of 1 per cent. In two series of tests, runs1 and 2, an acid corrosion inhibitor consisting of 1.3 per cent ofsodium arsenite was present in each test solution of 300 grams of 15 percent hydrochloric acid. In the other, run 3, the acid corrosioninhibitor consisted of about 1 per cent of crude quinoline bases knownin the trade as Barrett Inhibitor No. 93.

Table Corrosion rate in lbs. of metal dissolved per sq. ft. per day at150 F. in 15% H01 without and with added Fe and inhibitors Initialferric iron concentration Acid Corrosion 1111119 acid solution InhibitorRun No.

1 per cent Ferric ion corrosion inhibitor 2.77 2.97 NW 'rioi, 0rd,

1 1.3% of As as sodium ar- 0. 182 0. 534 0. 094

senite. 2 1.3% otl As as sodium ar- 0. 236 0. 630 0. 242

seni e. 3 1% of crude quinoline.-- 0. 055 0. 185 0. 126

Reference to the table shows that the nominal rate of corrosion of mildsteel by inhibited 15 per cent hydrochloric acid (0.182 pound per squarefoot per day in run 1, 0.236 in run 2, and 0.055 in run 3) is greatlyincreased when even a small amount (1%) of dissolved ferric iron ispresent in the acid solution. The increases for each of the two testswith sodium arsemte as the acid inhibitor but without the ferric ironinhibitor are 193 per cent and 167 per cent, respectively, and with thecrude quinoline 236 per cent. when a ferric iron corrosion inhibitor isalso present in the acid along with the acid corrosion inhibitor, thereis a large reduction in corrosiveness in spite of the presence of theadded ferric iron. In the case of run 1, the reduction in corrosivenesswas 82.4 per cent, in run 2 the reduction was 61.6 per cent, in run 3,46.8 per cent.

By the term non-plating metal used herein is meant a metal which doesnot deposit as a metallic film on an iron or steel object brought intocontact with an aqueous solution of one of its salts.

I claim:

1. A composition for descaling a ferrous metal surface comprising byweight 5 to 25 per cent of .hydrogmchlorigle, 0.1 to 1 per cent of anorganic nitrogen base selected from the group consisting of p l nquinoline, methyl quinoline, and otoluxdi'riEind 2.2 to 3.3 per cent ofa water-soluble salt selected from the group consisting of thewater-soluble divalent chromium salts and the water-soluble trivalenttitanium salts, the balance being water.

2. A composition for descaling a ferrous metal surface comprising byweight 5 to 25 per cent of hydrogen chloride, 0.4 to 1.0 per cent ofcrude quinoline, and 2.2 to 3.3 per cent of chromous chloride, thebalance being water.

3. A composition for descaling a ferrous metal surface comprising byweight 5 to 25 per cent of hydrogen chloride, 0.1 to 1.0 per cent ofcrude quinoline, and from 1.65 to 2.5 per cent of titanium trichloride,the balance being water.

HAROLD A. ROBINSON.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,371,584 Urruty Mar. 15, 19211,778,634 I-Iartmenn et al. Oct. 14. 1930 1,785,513 Calcott et al. Dec.16, 1930 1,877,504 Grebe et al. Sept. 13, 1932 2,357,991 Ayers Sept. 12,1944 2,384,467 Hill Sept. 11, 1945 FOREIGN PATENTS Number Country Date442,052 Great Britain J an. 31, 1936

